Windsor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windsor clear sky chart from 2005-11-06 to 2026-03-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	18% 12% 4 10% 6% 5% 4 3 5% 6% 27%	132364
	Night	17% 13% 4 11% 6% 5% 4 3 5% 6% 26%	129279
Transparency	Day	57% 5% 11% 18% 8% 1	135943
	Night	59% 5% 11% 17% 7% 1	133787
Seeing	Day	27% 21% 30% 16% 5% 1	19162
	Night	26% 18% 30% 21% 5%	26609

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 20% 6% 16% 10% 6% 3 3 3 4 15%	4690
	11th to 20th	Day	15% 20% 10% 21% 7% 5% 4 3 3 3 10%	4868
	21st to end	Day	12% 12% 6% 15% 10% 8% 6% 4 4 5% 19%	5781
February	1st to 10th	Day	16% 15% 6% 14% 9% 5% 5% 4 5% 5% 16%	1890
	11th to 20th	Day	16% 15% 6% 14% 8% 5% 3 4 5% 5% 20%	1843
	21st to end	Day	10% 15% 5% 10% 7% 6% 4 4 4 6% 29%	1576
March	1st to 10th	Day	21% 17% 5% 13% 6% 5% 3 3 3 3 21%	5039
	11th to 20th	Day	13% 17% 6% 9% 5% 4 4 3 5% 6% 27%	5057
	21st to end	Day	20% 19% 6% 12% 5% 5% 4 3 3 4 20%	5466
April	1st to 10th	Day	21% 16% 5% 9% 5% 3 4 3 5% 5% 25%	2385
	11th to 20th	Day	19% 13% 3 8% 4 4 4 4 5% 6% 32%	2457
	21st to end	Day	21% 13% 3 7% 5% 4 4 3 4 7% 29%	2600
May	1st to 10th	Day	27% 16% 3 8% 4 4 4 3 5% 6% 19%	6019
	11th to 20th	Day	22% 12% 3 8% 4 5% 4 3 5% 6% 29%	6318
	21st to end	Day	17% 12% 3 8% 5% 4 4 3 5% 8% 30%	6948
June	1st to 10th	Day	25% 9% 3 7% 5% 4 5% 4 5% 7% 26%	2800
	11th to 20th	Day	20% 9% 3 6% 5% 4 4 3 5% 7% 33%	2950
	21st to end	Day	23% 11% 3 7% 5% 4 4 3 5% 6% 27%	3000
July	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 9% 28%	6738
	11th to 20th	Day	13% 8% 3 8% 6% 5% 5% 4 6% 8% 35%	6360
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	6708
August	1st to 10th	Day	18% 10% 3 9% 5% 5% 5% 4 6% 7% 29%	4235
	11th to 20th	Day	14% 7% 3 8% 6% 5% 5% 4 6% 7% 35%	4175
	21st to end	Day	12% 6% 2 6% 4 4 4 4 6% 8% 44%	4510
September	1st to 10th	Day	16% 7% 2 5% 4 4 4 3 5% 7% 42%	2400
	11th to 20th	Day	17% 6% 2 6% 4 3 3 3 5% 6% 44%	2315
	21st to end	Day	17% 7% 2 5% 4 4 3 3 5% 7% 42%	2140
October	1st to 10th	Day	17% 6% 3 7% 5% 4 4 3 6% 7% 39%	2000
	11th to 20th	Day	18% 10% 3 7% 4 4 5% 4 5% 7% 33%	2000
	21st to end	Day	19% 13% 4 8% 6% 4 4 3 4 5% 30%	2200
November	1st to 10th	Day	20% 15% 3 7% 5% 5% 5% 2 4 5% 28%	1700
	11th to 20th	Day	19% 20% 6% 8% 5% 4 4 3 4 5% 22%	1680
	21st to end	Day	17% 18% 6% 11% 5% 5% 4 4 4 5% 20%	1680
December	1st to 10th	Day	19% 15% 9% 15% 6% 5% 4 3 4 4 15%	2690
	11th to 20th	Day	22% 15% 6% 13% 8% 5% 4 3 4 3 18%	3076
	21st to end	Day	18% 14% 8% 17% 6% 4 3 3 4 5% 18%	4070
January	1st to 10th	Night	14% 16% 7% 20% 9% 6% 4 3 4 4 14%	8234
	11th to 20th	Night	18% 17% 8% 19% 6% 5% 4 3 3 4 13%	8022
	21st to end	Night	11% 11% 5% 17% 14% 8% 5% 4 5% 5% 16%	8400
February	1st to 10th	Night	17% 15% 6% 15% 8% 6% 4 4 4 4 17%	2730
	11th to 20th	Night	16% 14% 5% 12% 9% 7% 5% 4 5% 5% 20%	2663
	21st to end	Night	11% 15% 5% 12% 7% 5% 4 3 5% 6% 28%	2227
March	1st to 10th	Night	21% 15% 5% 13% 9% 5% 4 3 3 4 20%	5020
	11th to 20th	Night	17% 17% 6% 8% 4 4 4 3 4 5% 28%	5057
	21st to end	Night	23% 16% 6% 11% 5% 3 3 2 3 4 24%	5466
April	1st to 10th	Night	22% 14% 5% 7% 5% 4 4 3 5% 4 27%	2014
	11th to 20th	Night	21% 12% 4 7% 3 4 4 3 4 6% 32%	1888
	21st to end	Night	22% 13% 2 6% 5% 4 4 4 5% 6% 30%	1800
May	1st to 10th	Night	24% 18% 3 8% 5% 4 4 3 4 4 22%	3926
	11th to 20th	Night	24% 10% 3 6% 4 4 4 3 5% 6% 31%	3596
	21st to end	Night	16% 13% 3 7% 5% 4 4 4 5% 7% 32%	3948
June	1st to 10th	Night	24% 9% 3 7% 4 4 4 3 6% 8% 29%	1600
	11th to 20th	Night	19% 8% 2 6% 5% 5% 4 3 6% 7% 36%	1464
	21st to end	Night	22% 9% 3 7% 6% 5% 5% 4 6% 7% 28%	1400
July	1st to 10th	Night	15% 10% 3 10% 6% 6% 5% 5% 6% 8% 26%	3178
	11th to 20th	Night	14% 6% 3 7% 4 5% 4 4 7% 9% 38%	3620
	21st to end	Night	12% 7% 2 6% 4 4 4 4 7% 9% 42%	4154
August	1st to 10th	Night	17% 9% 3 7% 4 4 4 4 5% 7% 35%	2915
	11th to 20th	Night	12% 8% 3 7% 6% 5% 5% 4 6% 8% 37%	2872
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 8% 49%	3388
September	1st to 10th	Night	12% 7% 2 5% 5% 4 4 3 5% 8% 45%	2000
	11th to 20th	Night	16% 8% 2 6% 3 3 3 3 5% 8% 44%	2082
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 6% 40%	2258
October	1st to 10th	Night	18% 7% 2 5% 4 4 5% 3 5% 7% 41%	2400
	11th to 20th	Night	18% 9% 2 6% 5% 4 5% 3 5% 8% 36%	2400
	21st to end	Night	23% 10% 3 7% 4 3 4 3 4 6% 31%	2640
November	1st to 10th	Night	20% 12% 4 7% 5% 4 4 3 5% 5% 31%	2814
	11th to 20th	Night	18% 20% 4 9% 5% 4 4 3 5% 5% 22%	2940
	21st to end	Night	20% 15% 5% 9% 6% 5% 4 3 4 6% 23%	2940
December	1st to 10th	Night	14% 13% 6% 17% 7% 6% 5% 4 5% 5% 18%	4720
	11th to 20th	Night	18% 16% 5% 12% 7% 5% 4 3 4 4 22%	5353
	21st to end	Night	22% 14% 6% 12% 7% 3 3 2 3 4 23%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	74% 5% 11% 8% 2	4691
	11th to 20th	Day	79% 5% 6% 8% 2	4898
	21st to end	Day	66% 5% 8% 10% 9% 2	5781
February	1st to 10th	Day	70% 4 8% 11% 6% 1	1890
	11th to 20th	Day	67% 4 9% 14% 6% 1	1851
	21st to end	Day	56% 5% 10% 17% 10% 2	1576
March	1st to 10th	Day	68% 3 7% 13% 8% 1	5043
	11th to 20th	Day	58% 4 13% 18% 6% 1	5057
	21st to end	Day	70% 3 4 14% 8% 1	5466
April	1st to 10th	Day	63% 3 6% 14% 11% 3	2385
	11th to 20th	Day	54% 4 8% 21% 12% 1	2457
	21st to end	Day	56% 4 8% 19% 11% 2	2600
May	1st to 10th	Day	66% 4 9% 14% 6% 1	6030
	11th to 20th	Day	58% 5% 11% 20% 6%	6322
	21st to end	Day	53% 5% 13% 18% 10%	6948
June	1st to 10th	Day	58% 5% 11% 19% 7% 1	2800
	11th to 20th	Day	50% 5% 12% 21% 11% 1	2950
	21st to end	Day	58% 5% 10% 17% 10% 1	3000
July	1st to 10th	Day	52% 6% 16% 16% 9% 1	6664
	11th to 20th	Day	47% 7% 14% 23% 9% 1	6284
	21st to end	Day	41% 9% 17% 24% 8%	6516
August	1st to 10th	Day	55% 6% 10% 19% 9%	4222
	11th to 20th	Day	46% 7% 14% 23% 9%	4214
	21st to end	Day	35% 7% 13% 27% 17% 1	4510
September	1st to 10th	Day	43% 4 11% 26% 15% 2	2400
	11th to 20th	Day	43% 4 11% 25% 16% 1	2315
	21st to end	Day	43% 4 9% 26% 17%	2140
October	1st to 10th	Day	43% 8% 13% 24% 12%	3260
	11th to 20th	Day	45% 6% 10% 26% 13%	3240
	21st to end	Day	48% 4 14% 22% 11%	3538
November	1st to 10th	Day	60% 5% 9% 19% 8% 1	1700
	11th to 20th	Day	64% 5% 11% 14% 5% 1	1680
	21st to end	Day	67% 6% 11% 13% 4	1680
December	1st to 10th	Day	70% 7% 8% 12% 2	2690
	11th to 20th	Day	71% 5% 6% 13% 4	3075
	21st to end	Day	71% 3 11% 11% 4	4070
January	1st to 10th	Night	72% 6% 9% 11% 2	8234
	11th to 20th	Night	76% 5% 7% 9% 3	8067
	21st to end	Night	68% 7% 9% 11% 5% 1	8400
February	1st to 10th	Night	71% 3 8% 11% 6% 1	2730
	11th to 20th	Night	65% 4 11% 14% 5% 1	2666
	21st to end	Night	59% 4 10% 18% 8% 1	2227
March	1st to 10th	Night	70% 3 6% 12% 9% 1	5029
	11th to 20th	Night	59% 4 12% 17% 7% 1	5058
	21st to end	Night	67% 3 6% 16% 7% 1	5466
April	1st to 10th	Night	61% 4 8% 14% 11% 3	2014
	11th to 20th	Night	55% 3 7% 21% 13% 1	1888
	21st to end	Night	57% 3 10% 16% 11% 2	1800
May	1st to 10th	Night	66% 3 10% 13% 7% 1	3932
	11th to 20th	Night	55% 4 10% 23% 7% 2	3597
	21st to end	Night	53% 5% 15% 21% 6% 1	3949
June	1st to 10th	Night	54% 5% 13% 21% 7% 1	1600
	11th to 20th	Night	49% 4 11% 22% 12% 1	1464
	21st to end	Night	56% 6% 13% 16% 8% 1	1400
July	1st to 10th	Night	55% 8% 14% 16% 7% 1	3143
	11th to 20th	Night	43% 7% 17% 22% 10% 1	3572
	21st to end	Night	39% 7% 17% 24% 12%	4032
August	1st to 10th	Night	49% 7% 15% 24% 6%	2903
	11th to 20th	Night	45% 7% 18% 23% 7%	2899
	21st to end	Night	32% 6% 18% 28% 16%	3387
September	1st to 10th	Night	38% 5% 12% 29% 14% 2	2000
	11th to 20th	Night	41% 4 11% 30% 14% 1	2082
	21st to end	Night	43% 5% 10% 28% 13% 1	2258
October	1st to 10th	Night	46% 6% 12% 24% 11%	3920
	11th to 20th	Night	41% 6% 12% 26% 14%	3899
	21st to end	Night	45% 4 14% 23% 13%	4252
November	1st to 10th	Night	55% 5% 12% 19% 9% 1	2814
	11th to 20th	Night	65% 4 10% 15% 5%	2940
	21st to end	Night	64% 4 14% 14% 3	2940
December	1st to 10th	Night	66% 7% 10% 14% 2	4720
	11th to 20th	Night	66% 5% 8% 15% 5% 1	5355
	21st to end	Night	67% 3 10% 16% 5%	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	29% 29% 29% 13% 1	430
	11th to 20th	Day	36% 24% 26% 11% 3	430
	21st to end	Day	27% 27% 33% 11% 2	473
February	1st to 10th	Day	26% 27% 31% 13% 2	430
	11th to 20th	Day	32% 30% 26% 11% 2	426
	21st to end	Day	25% 29% 31% 12% 3 1	355
March	1st to 10th	Day	30% 29% 31% 8% 2	540
	11th to 20th	Day	29% 30% 28% 10% 4	528
	21st to end	Day	33% 30% 23% 10% 3	550
April	1st to 10th	Day	38% 27% 27% 6% 2	500
	11th to 20th	Day	32% 29% 28% 9% 2	492
	21st to end	Day	30% 26% 26% 14% 4	500
May	1st to 10th	Day	34% 26% 26% 9% 4 1	558
	11th to 20th	Day	26% 20% 30% 18% 5% 1	724
	21st to end	Day	29% 17% 33% 16% 4 1	790
June	1st to 10th	Day	33% 16% 30% 15% 6%	730
	11th to 20th	Day	26% 15% 32% 21% 5% 1	730
	21st to end	Day	31% 15% 30% 18% 5% 1	730
July	1st to 10th	Day	23% 10% 36% 22% 8% 1	720
	11th to 20th	Day	20% 9% 36% 24% 9% 1	718
	21st to end	Day	22% 10% 34% 27% 6% 1	703
August	1st to 10th	Day	16% 10% 37% 28% 8% 2	526
	11th to 20th	Day	20% 11% 33% 21% 12% 3	519
	21st to end	Day	14% 18% 34% 21% 10% 3	583
September	1st to 10th	Day	18% 14% 35% 24% 8% 1	530
	11th to 20th	Day	25% 14% 28% 21% 8% 3	530
	21st to end	Day	21% 18% 31% 20% 8% 2	497
October	1st to 10th	Day	22% 17% 32% 18% 8% 3	420
	11th to 20th	Day	24% 22% 31% 14% 7% 2	420
	21st to end	Day	27% 26% 27% 15% 5% 1	462
November	1st to 10th	Day	30% 22% 29% 14% 4 1	425
	11th to 20th	Day	37% 22% 27% 9% 5%	430
	21st to end	Day	32% 27% 27% 10% 3 1	430
December	1st to 10th	Day	37% 23% 27% 10% 2	430
	11th to 20th	Day	36% 28% 25% 9% 1	430
	21st to end	Day	30% 28% 29% 9% 3 1	473
January	1st to 10th	Night	28% 25% 31% 14% 1	950
	11th to 20th	Night	29% 23% 30% 14% 3	912
	21st to end	Night	24% 27% 31% 15% 3	924
February	1st to 10th	Night	27% 24% 32% 15% 2	840
	11th to 20th	Night	31% 27% 27% 13% 2	823
	21st to end	Night	22% 26% 30% 18% 4	692
March	1st to 10th	Night	28% 25% 30% 15% 2	840
	11th to 20th	Night	25% 20% 32% 18% 4	829
	21st to end	Night	28% 20% 32% 16% 4	880
April	1st to 10th	Night	32% 21% 27% 16% 3	614
	11th to 20th	Night	30% 21% 30% 16% 3 1	594
	21st to end	Night	31% 18% 29% 17% 5%	600
May	1st to 10th	Night	30% 17% 30% 18% 4 1	600
	11th to 20th	Night	29% 13% 30% 22% 6% 1	592
	21st to end	Night	21% 14% 40% 22% 2	649
June	1st to 10th	Night	28% 14% 32% 24% 4	600
	11th to 20th	Night	22% 11% 36% 27% 5%	464
	21st to end	Night	24% 8% 30% 29% 9%	400
July	1st to 10th	Night	19% 7% 32% 32% 10%	395
	11th to 20th	Night	17% 6% 35% 33% 10%	582
	21st to end	Night	17% 7% 30% 36% 9%	656
August	1st to 10th	Night	16% 6% 30% 37% 11% 1	598
	11th to 20th	Night	18% 8% 26% 36% 12% 1	592
	21st to end	Night	17% 7% 29% 34% 13% 1	660
September	1st to 10th	Night	15% 10% 33% 31% 10% 1	600
	11th to 20th	Night	20% 9% 26% 32% 12% 1	682
	21st to end	Night	22% 13% 30% 26% 9% 1	800
October	1st to 10th	Night	22% 11% 31% 26% 10% 1	800
	11th to 20th	Night	23% 16% 33% 20% 8% 1	800
	21st to end	Night	32% 17% 29% 18% 4	880
November	1st to 10th	Night	30% 18% 28% 18% 5% 1	916
	11th to 20th	Night	32% 21% 26% 17% 4 1	950
	21st to end	Night	32% 21% 26% 17% 4	950
December	1st to 10th	Night	33% 20% 29% 15% 3	950
	11th to 20th	Night	32% 26% 28% 12% 2	950
	21st to end	Night	30% 25% 27% 15% 3 1	1045

Page generated at 2026-03-15 03:23:54. Version ac07

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